Until a few months ago, you might have never run across the name “Zika.” The virus is named for the Zika forest in Uganda where scientists first identified it in 1947.

Since then, scientists have figured out the basics for how the virus works. Zika spreads through mosquito bites, with symptoms like fever and joint pain that usually appear a few days later. Because of how it spreads and the nature of these symptoms, Zika is often compared to dengue fever, also called “breakbone fever.” But Zika is generally milder—only about one fifth of patients develop any symptoms, and most who do are back to normal within a week. So far, Zika has not caused any deaths. There’s no specific treatment or vaccine for Zika; those infected usually just need to take aspirin, drink water, and get lots of rest.

Researchers, however, have recently found a new, disturbing side effect. In October, epidemiologists studying the current outbreak in Brazil noticed a higher incidence of microcephaly, a rare neurological condition in which babies are born with smaller than normal heads and underdeveloped brains. Microcephaly is usually rare, but in some areas where Zika was particularly common, the condition was popping up many more times than it had in previous years–Brazil public health officials reported that in 2015, 3,000 babies were born with the condition, compared to 20 the year before. They noticed more babies with microcephaly were born to mothers infected with Zika, even if they didn’t show any symptoms.

Researchers are still not certain how Zika is triggering the increased incidence of microcephaly. But the connection is causing panic—so much so that the Brazilian government recently told women in the country’s northeast, where the epidemic has hit hardest, not to get pregnant for fear of these birth defects.